docs: def to_pickle(graph: BELGraph, file: Union[str, BinaryIO], protocol: int = HIGHEST_PROTOCOL) -> None:
      """Write this graph to a pickle object with :func:`networkx.write_gpickle`.
  
      Note that the pickle module has some incompatibilities between Python 2 and 3. To export a universally importable
      pickle, choose 0, 1, or 2.
  
      :param graph: A BEL graph
      :param file: A file or filename to write to
      :param protocol: Pickling protocol to use. Defaults to ``HIGHEST_PROTOCOL``.
  
      .. seealso:: https://docs.python.org/3.6/library/pickle.html#data-stream-format
      """
      raise_for_not_bel(graph)
      nx.write_gpickle(graph, file, protocol=protocol)